01 - Preheat the oven to 350°F. Line an 8x8-inch baking pan with parchment paper, leaving enough overhang on two sides to lift the brownies out later.
02 - In a heatproof bowl set over a saucepan of barely simmering water, melt the butter and chopped chocolate together, stirring occasionally until completely smooth. Remove from heat and let cool for about 5 minutes.
03 - Add the granulated sugar and light brown sugar to the melted chocolate mixture. Whisk until well blended and smooth.
04 - Add the eggs one at a time, whisking thoroughly after each addition until fully incorporated. Stir in the vanilla extract until the mixture is glossy and uniform.
05 - Sift the flour, cocoa powder, and salt together directly into the batter. Gently fold the dry ingredients into the wet mixture using a spatula until just combined. Be careful not to overmix, as this will toughen the texture.
06 - If desired, fold in the chopped walnuts or pecans and dark chocolate chips until evenly distributed throughout the batter.
07 - Pour the batter into the prepared baking pan and spread it into an even layer using a spatula. Smooth the surface to ensure uniform thickness.
08 - Bake on the center rack for 22 to 25 minutes. The brownies are done when the edges are set and the center has a slight jiggle. Avoid overbaking to maintain a fudgy interior.
09 - Allow the brownies to cool completely in the pan on a wire rack. Once fully cooled, use the parchment overhang to lift the slab out and cut into 12 even squares.